Standard ADCs transform analog voltage input signals by comparing them to a reference voltage signal. The voltage used by the ADC as reference defines the full scale range of the conversion range. Typically, the digital output signal Dout is defined by the ratio of the input voltage Vin to be converted and the reference voltage Vref:Dout=2n*(Vin/Vref),wherein n corresponds to the digital length of the output signal word and thereby also specifies the resolution of the conversion process. The equation represents the desired digital result as an expected output for some applications. As may be seen, the digital result Dout is based on a ratiometric value, i.e. a ratio, in this case the ratio between the input voltage Vin to be converted and the reference voltage Vref which is typically provided by the application and may have a value of 3.3V or 5V for example.
In standard ADCs used for digitizing an analog input value, an external reference voltage is applied to the ADC. This external reference voltage is often derived from the reference voltage of the application into which the ADC is embedded into. Typically, that reference voltage may be for example 3.3V or 5V, and it may be generated by an external voltage regulator. Consequently, when designing the ADC devices/components (such as transistors and capacitors) which have a rated operating voltage of 3.3V or 5V may need to be used such that the external reference voltage Vref can be fed to the ADC core without the ADC core being damaged or producing bogus results. In other words, the core of the ADC is required to be capable to handle 3.3V or 5V.
Due to the usage of 3.3V or 5V signals, those types of ADCs implemented in a deep sub-micron technology require devices in the ADC core which are able to handle 5V signals. This may lead to a high area consumption of the ADC as devices designed for higher voltages tend to be larger. In addition, the speed of an ADC circuit is mainly dictated by the performance of high voltage devices which is lower compared to digital core logic devices which may typically operate with operating voltages in the core voltage domain of for example 1.5V or less.